This invention relates to programmable logic devices and in particular to programmable logic devices containing a summing function and still more particular to programmable logic devices in which the summing function may be user configurable.
Programmable logic devices are user configurable integrated circuits. These devices contain a set of undedicated inputs, a set of undedicated outputs and a programmable array which permits the logic connection of these inputs to a given output. The most common implementation of this logic connection is a sum of products using an OR summing function, that is, with inputs A,B,C, and D, an output takes the form of; OUT=(A*B)+(C*D) where the terms (A*B) and (C*D) are called product terms. These product terms are generated from the programmable array and may include any of the inputs, or their complements, in each AND function. The logical representation of the sum-of-products with an OR summing function is exemplified by a device called AMPAL20R4 manufactured by Advanced Micro Devices of Sunnyvale, CA and illustrated on page 4-130 of the "Programmable Logic Handbook/Data Book" which was copyrighted in 1986 by Advanced Micro Devices, Inc.
A less common, but very powerful, approach to the summing function is the exclusive OR (XOR) implementation. The logical representation of this configuration is illustrated on page 4-170 of the above referenced handbook. The XOR sum can implement certain logical functions, such as counters, comparators, parity generators-checkers, more efficiently than the standard OR sum function.